1. Field of the Invention
The invention relates to a method of forming or performing secondary forming operations (hereinafter simply called "forming") on plastic films, sheets, plates, strips, or tubes (hereinafter called "sheets or the like") in the thermoelastic state by means of a heated molding or forming tool with high thermal conductivity. In the thermoelastic state, plastic sheets or the like and tubes can be formed in numerous ways, by bending, stretching, bowing, or blowing. The shapes obtained by forming are durably "frozen in" by cooling the material to below the softening temperature.
2. Discussion of the Background
The techniques of industrial forming are set forth in detail in Vieweg, R., and Esser, F., 1975, "Polymethacrylate" Vol. IX, pub. in Munich, pp. 550-573. Forming tools may be used for forming by blowing, vacuum forming, or stretching. They may be fabricated from wood materials, and are coated with a so-called "glove material" in order to avoid making impressions on the plastic undergoing forming (see ibid. p. 550). The low thermal conductivity of the wood is advantageous (p. 550). For large serial production, metal tools are preferred. These are coated with heat-resistant paints or molded resins, in order to improve their surface qualities and to provide thermal insulation (pp. 551 and 564). Polishing the metal surface to a high gloss does not result in preservation of the surface smoothness of the plastic during the forming, and in fact leads to pitting. It is preferable if the surface has a satin or matte finish (p. 551).
It is known to heat the surface of metal tools by means of heating elements incorporated in the tool (p. 551). For example, tools for blow mold forming and vacuum mold forming of acrylic glass substitutes are heated to 60.degree.-80.degree. C., in order to prevent cooling of the material (p. 564). For stretching acrylic glass substitutes by means of a drawing punch, it is recommended that the punch be manufactured of a material which is a good heat conductor and that the punch be heated to a high temperature, e.g. 80.degree.-110.degree. C. (p. 566), in order to avoid premature cooling of the plastic. The use of a relatively hot tool makes it possible to substantially, but not completely, avoid cooling streaks (p. 567). Heated metal tools also aid in avoiding stress formation in the formed piece due to nonuniform cooling (p. 568).
In all the known forming methods, even where heated tools are employed, the formed piece is essentially only disengaged from the forming tool after it has been completely cooled to a temperature below the softening temperature. Experience has shown that despite all the abovementioned precautionary measures, it is not possible to completely avoid impressions of the forming tool on the surface of the plastic. In particular, surfaces of high smoothness, such as are characteristic of sheets of acrylic glass substitutes, are readily damaged by even minor impressions, so that they look unattractive. The same applies to fine matte surfaces.
In order to avoid these problems in forming operations on thermoelastically softened plastic sheets or the like and in order to obtain a gloss, forming tools with a rigid body are used according to U.S. Pat. No. 4,704,081 and Eur. OS No. 201 044. The tools are coated with an elastic layer having a smooth, closed, matte surface of higher elasticity than that of the softened plastic sheet. In this case the forces active between the surface of the tool and the plastic result preferentially in deformation of the tool surface rather than the surface of the plastic. However, it is difficult to manufacture such tools if they are to have spherically curved surfaces.